1. Field of the Invention
This invention relates to a hydraulic control system for controlling the operation of a variable camshaft timing (VCT) system. More specifically, the present invention relates to a control system which utilizes a dual pulsed width modulated solenoid or a four-way valve to control a cam phaser.
2. Description of Related Art
U.S. Pat. No. 4,627,825 uses two electromagnetic solenoids, each operating a valve to move a phaser in one direction or the other. The pressure moves the phaser directly.
U.S. Pat. No. 5,150,671 uses an electromagnetically operated external spool valve to to supply switched hydraulic pressure to activate a central spool valve. The external valve is a two-way PWM valve.
U.S. Pat. No. 5,333,577 teaches closed loop control of a spool valve using an electromagnetic linear solenoid. This patent describes a strategy for computing solenoid position based on deviation from desired angle and temperature.
U.S. Pat. No. 5,363,817 teaches a control strategy to avoid operational variations.
U.S. Pat. No. 5,666,914 shows a vane phaser which has pilot valves in the rotor.
Consideration of information disclosed by the following U.S. Patents, which are all hereby incorporated by reference, is useful when exploring the background of the present invention.
There are many ways to control the position of a spool valve that controls the oil flow to and from the chamber of a vane or piston style cam phaser. These control methods include an external mounted solenoid DPCS (differential pressure control system), shown in U.S. Pat. No. 5,107,804, a variable force solenoid, shown in U.S. Pat. No. 5,497,738, and a stepper motor, shown in U.S. Pat. No. 5,218,935.
Although the variable force solenoid reduces the dependency of the control system on the oil pressure from the engine and eliminates the need to have a spool with different diameters, it does need to be mounted in front of the cam phaser and causes the length of the engine to increase. The VFS pushes on one end of the center mounted spool valve against a spring that will return the valve to a default and fail-safe position when the solenoid is off.
The stepper motor system also increases the length of the engine as it is mounted in front of the cam phaser. This system has trouble with the fail-safe positional control of the phaser. The position of the stepper motor will not return to a fail-safe position once it is turned off.
The present invention includes a remotely mounted 4-way valve or two solenoid valves to control a center mounted spool valve. In the 4-way valve embodiment, one control port provides oil pressure to one end of the spool valve and the other control port provides oil pressure to the other end of the spool. In the embodiment with two solenoid valves, one solenoid valve control port feeds oil to one end of the spool and another solenoid valve control port feeds oil to the other end. With these systems, the two control pressures are always a percentage of the engine oil pressure. For both of these control systems, the relationship of percent of control signal to percent of control pressure is mapped into the controller, and can vary as the engine oil pressure and temperature changes. One method to reduce this error is to have a position sensor mounted to the spool valve position and have a control loop controlling the position of the spool valve. There is also another loop to control the phaser angle.